I am baffled that a full frame camera is being thrown around. I would suggest getting good glass (a lot of glass cost more than a camera body) and using a crop sensor to learn on.
Someone said an XS has a better sensor than an XSi. Uh, yeah. The XSi has one of the best sensors in any Rebel made. If you know where I can get an XS with a 50mm 1,8 for $100, please let me know. I would buy one in a heartbeat.
I don't know about 100, but a friend of mine sold his for 200$. XS body and 1.8 nifty fifty lens. Only used around 2 years.
I was telling guy up there that he should learn on a crop and it would make him want a full frame, especially when he gets good glass.
I think if you use the XS for a bit you won't ever think, "Oh I wish I had the XSi." Get the XS, save some dough and get some good glass.
Just my opinion.